W. Vernon

The electron stationary picture method for high-speed measurement of reflection intensities from crystals with large unit cells

A method for measuring X-ray intensity data from crystals with large unit ceils is presented. The method takes full advantage of the capabilities of the multiwire area detector diffractometer. This diffractometer is a high-speed data collection system utilizing a multiwire proportional chamber that can detect photons from all simultaneously occurring reflections and record the diffraction pattern as a two-dimensional histogram in a computer mass core memory. In the electronic stationary picture method of data collection, reflection intensities are extracted from a sequence of electronic pictures each of which is exposed while the crystal is stationary. Between successive pictures the crystal is rotated about a fixed axis by a small constant angle of approximately 0.05°. Because the integrated intensity of each reflection is extracted from several consecutive pictures, advance prediction of detector coordinates and picture number is required for all reflections and the data-extraction computer program is necessarily complex. Expressions are derived for reflection detector coordinates and setting angles, the system hardware and software are described, the data collection procedure is outlined, and the quality of 7 × 105 reflection intensities measured during six months of operation is analyzed.

A multiwire proportional chamber as an area detector for protein crystallography

A multiwire proportional chamber (30 × 30u2005cm) together with its electronic readout into a large core memory (mass core) has been used successfully as a digital area detector for protein crystallography. The diffraction pattern stored in the mass core can be displayed on a TV monitor. An IBM 1800 computer has fast random access to the mass core and is used on line to estimate the integrated reflection intensities. To characterize this new area detector, the geometric linearity, the resolution and the quantum detection uniformity have been measured. Preliminary results show that with this new system one can collect intensity data from protein crystals about an order of magnitude faster than with the standard diffractometer.

Characteristics of a flat multiwire area detector for protein crystallography

A flat, relatively thin (9 mm) xenon-filled multiwire proportional counter with two-dimensional, 2/~s delay line readout of a 270 x 300 mm active area has been developed for use as a position-sensitive area X-ray detector in the 8 keV energy region (Cu Ka) used in crystallographic structure work with large biological molecules. Its quantum detection efficiency for 8 keV Xray photons is about 0.5, a value which is spatially uniform to within _+ 2%. Its dead-time loss fraction at a typical data collection rate of 30 000 photons s-1 is 12%. The detector has spatial resolution for X-rays of 0.6 mm FWHM in the horizontal direction and 2 ram, the anode wire spacing, in the vertical direction. The effects of parallax are found to be limited and do not seriously increase the apparent size of the diffracted beams. The position sensitivity of this detector is geometrically linear to within 0-5 mm across its active surface. Routine maintenance of the detector requires the attention of a skilled technician but is not time consuming. For four years, this detector has been used to measure millions of reflection intensities from crystals of many different proteins. The down time due to the detector has averaged less than four days per year, considerably less than the down time of other components of the data collection system. Four new protein structures have now been solved using data from this detector. Also, a considerable amount of data have been collected at higher resolution or at different temperatures with crystals of other proteins.

First results from the 128x128 pixel mixed-mode Si x-ray detector chip

A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays are stopped in a 500 μm thick layer of silicon diodes, and collected charge is processed by an attached ASIC. Goals of the project are high flux (108 x-rays/s/pixel) capability and fast readout (< 0.5 ms dead time between frames). Mixed-Mode refers to a readout method whereby integrated signal accumulating in each pixel is compared against a threshold value. When the threshold is reached, a digital count is added to an 18-bit in-pixel counter and a set quantity of charge is removed from integrator. At the end of the x-ray exposure, analog signal left in the integrator is separately processed. Thus, one obtains mixed digital and analog data where the counter bits are a high order word and the analog residual provides higher precision. Typically, each count is equivalent to 100 10 keV x-rays, for a well-depth >107 10 keV x-rays/ pixel. The analog residual is digitized to 9-bit precision allowing measurement of the residual charge to better than a quarter of the charge from single 10 keV x-rays. Measurements are presented on x-ray tests at the Cornell High Energy Synchrotron Source (CHESS). Dynamic range, linearity, point-spread function and noise properties are shown. Status will be is reported on five different approaches for ASIC-diode hybridization. Progress toward bonding of a 128 x 512 pixel device is also presented.

Progress report on the development of an area detector data acquisition system for X-ray crystallography and other X-ray diffraction experiments

Abstract A relatively inexpensive data acquisition system, incorporating a flat MWPC, with an anticipated count rate of approximately 130 kHz is currently being developed at SSRL in collaboration with research groups at UCSD and LBL. It will be primarily used for X-ray crystallographic studies of macromolecules using synchrotron radiation but will also be used for other X-ray diffraction experiments at SSRL. This system, controlled by a PDP 11-34 computer running under the RSX-11M operating system, incorporates the MWPC area detector, a fast display system, a 3-circle single-crystal goniometer, an alignment carriage, an incident-beam intensity monitor and a tunable monochromator. A progress report and future plans will be presented. The detector dimensions are 28 cm × 28 cm with a spatial resolution of 128 × 256 elements. The absorption depth at normal incidence is 1 cm and the fill gas is a mixture of 90% Xe and 10% CO 2 at a pressure of 1 atmosphere. Fast printed-circuit delay-lines (≈ 170 ns) are used for read-out. It is anticipated that the complete system will be ready for initial testing in the Spring of 1980.

A xenon-filled multiwire arc detector for X-ray diffraction

A xenon-filled multiwire proportional chamber with high and uniform photon-detection efficiency has been built and incorporated into a high-speed data-collection system for protein crystallography. This system has been used to collect more than 200 000 reflection intensities from one parent and four heavy-atom derivative crystals of a complex of dihydrofolate reductase and methotrexate (a cancer drug). Its data collection rate is about 30 times faster than the rate of a standard diffractometer (1400 reflection intensities per hour versus 50 reflection intensities per hour). The precision of the chamber data is also better with an intensity reliability R of 5% as compared with 6.7% from the diffractometer data. The difference Patterson maps between parent and heavy-atom derivative data show very clearly the positions of the heavy atoms.

S - wave pion - pion scattering length from reaction π− + p → πo + πo + n at 378 MeV

Abstract The differential energy distributions of neutrons from the reaction π − + p → π o + π o + n were measured at four different neutron lab angles. Fitting the distributions with the Chew-Mandelstam solution for π − π phase shift, we obtain a value a SO = (0.28 ± 0.21) μ −1 for the S-wave π − π scattering length.

Leptonic structure functions of the photon

An analysis of the production of muon pairs in γγ collisions is presented. The leptonic structure functions of the photon, F2 and 2xF1, are extracted for Q2 < 1.7 GeV2. Their x dependences agree well with QED predictions. Corrections are required to eliminate radiative effects in the 2xF1 data.

Development of a Mixed-Mode Pixel Array Detector for macromolecular crystallography

A Mixed-Mode Pixel Array Detector (MMPAD) has been conceived for protein crystallography applications. The most notable feature of the detector is the combination of analog and digital approaches used in the pixel to generate very large dynamic range, low dead time, and fast readout of the types of spot intensity measurements needed in X-ray images from large macromolecules. Prototype structures for the pixel have been fabricated and evaluated and indicate that an intensity range of 10/sup 7/ to 1 can be measured with a precision of 0.25%, while still maintaining a high DQE for X-rays in the 6-18 keV range. The mixed-mode structure is compatible with a fast I/O system and allows the possibility of full-frame readout in the ms range. We present details of pixel behavior, design of support electronics, hybridization results and plans for fabrication of large arrays.

A study of double tagged two-photon reactions at spear☆

Abstract We have identified 262 doubly tagged two-photon events. A subset of the data shows an enhancement of 21 events in the inclusive two-photon mass squared distribution between 0.8 and 2.2 GeV 2 . If these events result from spin 2 resonance production then Γ γγ = 9.5 ± 3.9 ± 2.4 keV (statistical and systematic). From another subset of 58 events in which the final state could be classified we determine the two-photon hadron to muon cross section ratio R γγ = 1.1 ± 0.3 ± 0.3.